Fossil Footprints Show Movements of Our Early Ancestors

The trace fossils found in Tanzania spurred a debate about how early hominids lived

In 1978, researchers in Laetoli, Tanzania, found an 88-foot-long trail of preserved footprints. The 3.6 million-year-old imprints included 70 impressions from two ape-like early human ancestors, likely Australopithecus afarensis, and are the earliest set of bipedal footprints ever discovered. Now, researchers assessing the area of the famous tracks for a proposed museum have discovered another large set of bipedal prints, reports Helen Briggs at the BBC.

The new fossil prints, as well as the previously discovered tracks, were formed when the early hominids walked across a stretch of wet volcanic ash that hardened into a clay-like material, preserving the impressions. Ian Sample at The Guardian reports that preliminary analysis of the prints indicates that the newly found 90-foot stretch of tracks were made by a male, three females and a child walking as a group through the ash.

The largest footprints in the group show that the male Australopithicus likely stood five feet, five inches tall, making him the largest member of the species ever recorded, Sample reports. Lucy, the famous skeleton and first member of the species found, for instance, was only around 3.6 feet tall.

The composition of the group also adds to our scant knowledge of the species behavior. “A tentative conclusion is that the group consisted of one male, two or three females, and one or two juveniles, which leads us to believe that the male—and therefore other males in the species—had more than one female mate,” Marco Cherin, director of the school of paleoanthropology at the University of Perugia, tells Briggs. That may mean the social structure of the species may have resembled apes, such as gorillas, where a dominant male and several females from one child-rearing unit.

“We are far from the traditional representation of the 1970s, with a couple of human-like Australopithecus, romantically walking arm in arm,” Giorgio Manzi, of Italy’s Sapienza University of Rome and a co-author of a study on the footprints in the journal eLife tells Michael Greshko at National Geograhpic. “This old representation is probably misleading.”

But not everybody is comfortable with the team’s interpretation of the footprints, or the extrapolation that one of the creatures was so large. “The size variation they report has no bearing on sexual dimorphism, since we don’t know the age of any of the footprint makers,” Kent State’s Owen Lovejoy, who worked on the original Lucy fossil, tells Greshko. “The suggestion that somehow these five prints suggest a gorilla-like sexual strategy is—well, let me put it this way—nuts.”

It’s not the only controversy that has cropped up surrounding Australopithcus this year. Sample points out that researchers reanalyzing Lucy’s bones concluded that she had fractures consistent with falling out of a tree. Other researchers argued that an animal stampede or many other causes could have caused the breaks post-mortem. In November, researchers also argued that Lucy’s forearms were very strong for her size, indicating that she likely did spend time in trees.

Whatever the case, there’s likely to be more information to come from Laetoli. Manzi tells Greshko that the new footprints were found in three small trenches and future excavations may reveal many more remnants of the hominids who once passed that way.

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About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.